* Violence upsurge only weeks after successful Olympic bid

* More than 40 dead in week since police 'copter shot down

* Experts see need for profound security policy changes

By Stuart Grudgings

RIO DE JANEIRO, Oct 25 (Reuters) - The crackle and boom of automatic weapon fire greeted police as they entered Rio de Janeiro's Vila Cruzeiro slum, marking the start of an intense firefight with gangsters that lasted through the morning.

The police's armored cars, known as "big skulls" and studded by bullet marks, roared through the slum's streets as residents cowered behind walls and in doorways, their lives once again disrupted and endangered by Rio's drug war.

"It does scare me but I'm used to it," said 11-year-old Vito Ricardo, who was taking shelter in a doorway and said the firefight on Friday had kept him from going to school. Shortly afterward, a bullet slammed into a building nearby.

In just a few weeks, the image of the Brazilian city has lurched from scenes of joyful revelers on Copacabana beach soaking up the victorious bid to host the 2016 Olympics to ones of a city embroiled in a bloody war with itself.

Since suspected drug traffickers shot down a police helicopter last Saturday, killing three officers, police have launched their biggest offensive against the city's drug gangs in years. They invaded more than 10 slums, known as favelas, on some days last week.